This invention relates to a Coriolis flowmeter that measures a flow of process material having an ultra high level of purity.
It is known to use Coriolis effect mass flowmeters to measure mass flow and other information pertaining to materials flowing through a pipeline as disclosed in U.S. Pat. No. 4,491,025 issued to J. E. Smith, et al. of Jan. 1, 1985 and Re. 31,450 to J. E. Smith of Feb. 11, 1982. Flowmeters have one or more flow tubes of a straight, curved or irregular configuration. Each flow tube has a set of natural vibration modes which may be of a simple bending, torsional, or twisting type. Each material filled flow tube is driven to oscillate at resonance in one of these natural modes. The natural vibration modes are defined in part by the combined mass of the flow tubes and the material within the flow tubes. If desired, a flowmeter need not be driven at a natural mode.
Material flows into the flowmeter from a connected material source on the inlet side. The material passes through the flow tube or flow tubes and exits the outlet side of the flowmeter.
A driver applies force to oscillate the flow tube. When there is no material flow, all points along a flow tube oscillate with an identical phase in the first bending mode of the flow tube. Coriolis accelerations cause each point on the flow tube to have a different phase with respect to other points on the flow tube. The phase on the inlet side of the flow tube lags the driver; the phase on the outlet side leads the driver. Pickoffs are placed on the flow tube to produce sinusoidal signals representative of the motion of the flow tube. The phase difference between two sensor signals is divided by the frequency of oscillation to obtain a delay which is o proportional to the mass flow rate of the material flow.
It is known to use flowmeters having different flow tube configurations. Among these configurations are single tube, dual tube, straight tube, curved tube, and flow tubes of irregular configuration. Most of the flowmeters are made of metal such as aluminum, steel, stainless steel and titanium. Glass flow tubes are also known.
The positive attributes of titanium in flowmeters are its high strength and low coefficient of thermal expansion (CTE). The negative attributes of titanium are its metallic properties and cost of manufacturing. In semiconductor wafer processing, metal ions are a contaminant. Metal ions in contact with the wafer areas of an integrated circuit can cause a short circuit and ruin the device. Also, a Titanium flowmeter is difficult and expensive to produce.
The prior art also suggests plastic flow tubes and plastic flowmeters. This includes prior art in which the entirety of the flowmeter is plastic as well as that in which only the flow tube is formed of plastic. Much of this prior art merely contains an assertion that a flowmeter may be made of various materials such as steel, stainless steel, titanium or plastic. This prior art is not instructive in so far as concerns the disclosure of a plastic Coriolis flowmeter that can accurately output information over a range in operating conditions including temperature.
The mere substitution of a plastic flow tube for a metal flow tube will produce a structure that looks like a flowmeter. However, the structure will not function as a flowmeter to generate accurate output information over a useful range of operating conditions. The mere assertion that a flowmeter could be made out of plastic is nothing more than the abstraction that plastic can be substituted for metal. It does not teach how a plastic flowmeter can be manufactured to generate accurate information over a useful range of operating conditions.
It is a problem in some applications that the typical Coriolis flowmeter may contaminate the process material. This is undesirable for systems in which material of an ultra high level of purity must be delivered by the flowmeter to a user application. This is the case in the fabrication of semi-conductor wafers which requires the use of a process material that is free of contaminants including ions migrating from the tubes of the process material flow path. In such applications, the flow tube can be a source of contaminants. The metal walls of a flow tube can release ions into the process material flow. The released ions can cause the chips on a semi-conductor wafer to be defective. The same is true for a glass flow tube which can release the lead ions from the glass into the process material flow. The same is also true for the flow tubes formed of conventional plastics.
A plastic termed PFA is free from this objection since the material of which it is composed does not release deleterious ions into the material flow. The use of PFA for a flow tube is suggested in U.S. Pat. No. 5,918,285 to Vanderpol. This suggestion is incidental to the Vanderpol disclosure since the patent discloses no information regarding how a flowmeter having a PFA flow tube could be manufactured to generate accurate flow information.
Flow tubes lined with PFA, as disclosed in U.S. Pat. No. 5,403,533 to Dieter Meier, attempted to combine the positive attributes of both metal and plastic technologies but encountered new challenges that could not be solved until the present invention. Metal flow tubes lined with PFA allow metal ions to migrate through the thin PFA coating layer and into the flow stream to cause contamination. Also, the metal flow tube material and the PFA liner have different thermal properties. This caused the PFA liner to disengage from the flow tube to create leaks and performance problems. The manufacturing process for lining the metal flow tubes with PFA is also extremely costly.
The above and other problems are solved and an advance of the art is achieved by the present invention which discloses a Coriolis flowmeter having at least one flow tube formed of perfluoroalkoxy copolymer (PFA) plastic. The flow tube is coupled to a driver and to at least one pickoff sensor to enable the PFA flow tube to function as part of Coriolis flowmeter that can provide accurate output information over range of operating conditions for a process material flow of ultra high purity suitable for use in applications such as semi-conductor fabrication and the like which require the material flow to be free of contaminants down to the ionic level.
A flow path constructed entirely of PFA has many of the benefits of Titanium and PFA lined flow tubes without the drawbacks. PFA is a fluoropolymer with superior chemical resistance, little metal ion release, low particle generation, and is manufacturable without expending large amounts of capital. PFA material is strong and can be extruded into high quality thin wall tubing. Thin-walled PFA tubing has low flexural stiffness enabling a higher sensitivity to mass flow rate and improved immunity to elastic dynamic interaction between the flow tube and the process pipeline. The material and physical properties of PFA allow larger tube vibration amplitudes at lower stress levels and result in near infinite fatigue life span. Also, the higher vibration amplitude allows the use of small low-mass transducers, which in turn improves density sensitivity and immunity to mount variation.
A first preferred exemplary embodiment of the invention comprises a flowmeter having a single PFA plastic flow tube coupled to a massive metal base which balances the end node vibration of the flow tube. The base is U-shaped and the plastic flow tube extends through coaxial holes in the two legs of the U. The plastic flow tube is affixed to the holes in the base by means of an appropriate adhesive such as cyanoacrylate also termed Loctite 420. The longitudinal center of the flow tube is affixed to an electromagnetic driver which receives a drive signal from a meter electronics to vibrate the flow tube transversely to the longitudinal axis of the flow tube. This vibration may be at the first bending resonant frequency of the flow tube. The flow tube is coupled to pickoffs which detect the Coriolis response of the vibrating flow tube with material flow. In the first embodiment, the pickoffs may be a conventional electromagnet combination with magnets affixed to the flow tube and a coil affixed to the base. In an alternative embodiment, the pickoffs are optical devices which generate a light beam and that is modulated by the vibrations of the flow tube. The optical sensing embodiment offers the advantage that the weight of the pickoff magnets is removed from the vibrating flow tube. This increases density sensitivity. The driver is a source of heat that can raise the temperature of the plastic; thermally expand the plastic and degrade the accuracy of the generated output information. In this embodiment, the driver is advantageously affixed on top of the flow tube when in use. This mounting arrangement has the advantage that the heat generated by the driver radiates upwardly away from the flow tube.
In accordance with another embodiment, the magnets associated with the driver and sensor pickoffs have low mass since they are small and do not have keepers or pole pieces. The magnets and coils have been optimized to make the magnets as small (low mass) as possible. The magnet material has been chosen to have the most magnetic field per unit mass as possible. The tube geometry has been designed to achieve the desired motion with as little drive force as possible. PFA has naturally low damping, so drive force is inherently low due to selecting this material for the flow tube. All of these factors contribute to achieving low mass transducer parts on the flow tube. This is advantageous since it reduces the physical loading of the flow tube and enhances the output accuracy of the flowmeter.
The single flow tube comprises an unbalanced structure whose vibration is minimized by the massive base. The ratio of the mass of the base to mass of the single plastic flow tube together with its magnetic material mass is in the order of 3,000 to 1. This results in a heavy base structure having a weight of approximately 13 pounds for a flow tube, plus magnets and material mass having a total weight of about 2 grams. Although the 13 pound weight minimizes the vibrations at the nodes of the flow tube, it has a disadvantage of increasing the weight of the equipment of which the flowmeter is a part. These vibrations may be minimized by the use of a dynamic balancer or a active dynamic balancer. The active dynamic balancer transmits signals to the meter electronics which analyzes the signals and returns a control signal to the active dynamic balancer to reduce the undesired vibrations. This has the advantage that the overall weight of the base structure may be reduced from 13 pounds down to about 2 pounds.
As mentioned, the first preferred embodiment comprises a Coriolis flowmeter having a single straight tube operating in an unbalanced mode in cooperation with a massive base. Other flow tube configurations are provided by other embodiments of the invention. The invention may be practiced with the use of dual flow tubes vibrating in phase opposition. These dual tubes may either be straight, they may be u-shaped, or they may be of a irregular configuration. The use of dual flow tubes is advantageous in that it provides a dynamically balanced structure that reduces the mass of the base required to mount the flow tubes.
An additional embodiment that can be associated with any tube configuration is the provision of a temperature measurement device. A preferred embodiment is the use of a Resistive Temperature Device (RTD) attached to a flow tube. If desired, the temperature can be measured using an infrared temperature measurement device. The benefits to this device is that it is non-contact and can be located off the tube, thereby reducing mass on the tube. Also, the RTD can be mounted to another flow carrying tube in the sensor which is not the vibrating flow tube.
Another embodiment comprises a massive base having upwardly extending sidewalls and a single flow tube extending through coaxial holes in the sidewalls. The base has an inner and an outer pair of upwardly extending walls. The inner walls contain the stationary vibrational nodes of the active portion of the flow tube; the outer walls mount connectors to which an inlet of the flow tube is connected at one end and an outlet of the flow tube is connected at the other end. This arrangement provides a dynamically unbalanced structure comprising a single flow tube with any vibrations at the nodes of the active portion of the flow tube being suppressed by the inner pair of upwardly extending walls.
Still another embodiment of the invention comprises a massive base having upwardly extending side walls and a pair of flow tubes extending through holes in the side walls. The two flow tubes are connected in series in so far as concerns the process material flow. This connection is accomplished by means of short u-shaped length of PFA tubing at one end of the base. This short u-shaped length of tubing connects an outlet end of the first flow tube to an inlet end of the second flow tube which is positioned in the base parallel to the first flow tube. With this arrangement, an inlet end of the first flow tube and an outlet end of the second flow tube are positioned in the same upwardly extending wall of the massive base. The two flow tubes are vibrated by separate drivers in phase opposition. Each flow tube also has its pair of pickoffs for detecting the Coriolis response of its flow tube with material flow.
In summary, the Coriolis flowmeter embodying the present invention is advantageous in that it provides for the measurement and delivery of a process material having an ultra high level of purity. This level of purity is provided by the use of a PFA plastic flow tube which is superior to metals, glass and ordinary plastics all of which permit ion transfer from the flow tube material to the processed material. The processed material may typically comprise a slurry which is an organic compound used as a polishing agent in the fabrication of wafers in the semi-conductor industry. This polishing operation provides a flat surface for the wafers. The polishing operation can take about an hour during which time the slurry must be free from any contaminants. The deposit of a single undesired ion onto a semi-conductor wafer can short circuit all or a portion of the wafer and render it useless.
An aspect of the invention is a Coriolis flowmeter for measuring a process material flow having an ultra high level of purity, said Coriolis flowmeter comprising:
a base;
flow tube apparatus adapted to receive said process material flow, said flow tube apparatus is formed of a material that does not transfer ions from said flow tube apparatus to said process material;
end portions of said flow tube apparatus are coupled to said base to create substantially stationary nodes at said end portions;
said flow tube apparatus has high flexibility and also has a stiffness substantially lower than flow tube apparatus formed of metal or glass;
a driver coupled to said flow tube apparatus for vibrating said flow tube apparatus containing said process material flow;
pickoff means coupled signalwise to said flow tube apparatus for generating signals representing induced Coriolis deflections of said vibrating process material filled flow tube apparatus; and
meter electronics that receives said signals from said pickoff means and generates output information pertaining to said process material flow.
Preferably said flow tube apparatus defines a substantially straight single flow tube.
Preferably the entirety of the wetted flow path of said Coriolis flowmeter comprises a PFA substance.
Preferably said flow tube apparatus defines more than one flow tube.
Preferably said pickoff means is an electro-magnetic device having a magnet connected to said flow tube apparatus and further having a coil.
Preferably said pickoff means comprises a light source and an optical detector;
said vibrating flow tube apparatus is positioned between said light source and said optical detector to alter the characteristics of the light received by said optical detector from said light source;
said optical detector is responsive to said alteration to generate said signals representing said Coriolis deflections.
Preferably said base has a lower surface and an inner pair of upwardly extending side walls as well as an outer pair of upwardly ending walls;
openings in each of said upwardly extending walls are coaxially aligned to receive said flow tube apparatus.
Preferably said base is substantially u-shaped and has a lower surface and a pair of upwardly extending walls proximate sides of said base;
openings in each of said upwardly extending walls are coaxially aligned to receive said flow tube apparatus.
Preferably ends of said flow tube apparatus extend beyond said walls.
Preferably said base is a solid rectangular element defining a parallelepiped;
said flow tube apparatus is connected to posts affixed between said walls to a top surface of said base.
Preferably an inlet of said flow tube apparatus receives said process material flow from a supply tube;
an outlet of said flow tube apparatus is coupled to an inlet of a return tube;
said return tube is coupled to said base and is positioned parallel to said flow tube apparatus and extends through walls of said base, and
said return tube is adapted to be connected to an exit tube to extend said process material flow towards a user application.
Preferably said flow tube apparatus comprises a single flow tube and that said base has a mass substantially greater than the mass of said flow tube with process material.
Preferably the mass of said base is at least 1000 times the mass of said single flow tube with process material.
Preferably the mass of said base is at least 100 times the mass of said single flow tube with process material.
Preferably said driver is affixed to the top of said single flow tube when in use.
Preferably a dynamic balancer means coupled to said base proximate said nodes to maintain said nodes at a reduced level of vibration.
Preferably said dynamic balancer means is an active dynamic balancer controlled by the exchange of signals with said meter electronics.
Preferably said base is substantially u-shaped and has a lower surface and a pair of upwardly extending side walls containing coaxially aligned openings for receiving said single flow tube.
Preferably said single flow tube extends through coaxial openings in said walls.
Preferably said flow tube apparatus comprises a first and a second flow tube coupled to said base and positioned parallel to each other, said first and second flow tubes are adapted to be vibrated in phase opposition by said driver.
Preferably said driver is affixed to both said first flow tube and said second flow tube and is adapted to vibrate said first and second flow tubes in phase opposition;
said pickoffs being affixed to both said first and second flow tubes to detect the Coriolis deflections of said first and second flow tubes.
Preferably said first and second flow tubes are connected in series with respect to said material flow.
Preferably said first and second flow tubes are connected in parallel with respect to said material flow.
Preferably a return tube coupled to said base and oriented parallel to said first and second flow tubes;
said return tube receives said process material flow from said first and second flow tubes and extends said material flow towards a user application.
Preferably said base is u-shaped and has upwardly extending walls;
said first and second flow tubes extend through said walls of said base and have inlet and outlet ends projecting beyond the outer surfaces of said walls.
Another aspect comprises a Coriolis flowmeter for measuring a flow of process material having an ultra high level of purity;
said Coriolis flowmeter comprising:
a single flow tube formed of a material, such as PEA, that does not transfer ions from said single flow tube to said process material;
said single flow tube has high flexibility and further has a stiffness substantially lower than a metal or glass flow tube;
the entirety of the wetted path of said Coriolis flowmeter comprises said PFA material;
a driver affixed to said single flow tube for vibrating said single flow tube containing said process material flow;
a massive base affixed to ends of said single flow tube to absorb undesired vibratory forces generated by said vibrating flow tube;
said base defines stationary nodes at ends of said flow tube;
an inlet connector connected to said massive base and adapted to receive a flow of said process material from a supply tube;
an inlet end of said single flow tube is affixed to said inlet connector;
said input connector sealably connects said inlet end of said single flow tube to an outlet end of said supply tube to effect the extension of said process material flow in said supply tube to said single flow tube;
said inlet connector maintains said inlet end of said flow tube fixed with respect to said massive base;
an outlet end of said single flow tube affixed to a second connector for extending said process material flow via an exit tube towards a user destination;
a pair of pickoffs coupled to said single flow tube on opposite sides of said driver for generating signals representing Coriolis induced deflections of said vibrating material filled single flow tube;
meter electronics; and
conductors extending signals from said pickoffs to said meter electronics;
said meter electronics receives said pickoff output signals and generates output information pertaining to said process material flow.
Preferably a return tube connected to said massive base parallel to said single flow tube;
end portions of said single flow tube and said return tube are glued to said massive base to maintain said single flow tube and said return tube immovable with respect to said massive base;
an inlet of said return tube;
an intermediate tube connecting said outlet end of said single flow tube and said inlet end of said return tube via said second connector to extend said process material flow from said outlet end of said single flow tube to said return tube;
an outlet connector connected to said massive base for receiving said flow of said process material from said outlet end of said return tube;
said outlet connector sealably connects said outlet end of said return tube to an inlet end of an exit tube to effect the extension of said process material flow in said return tube to said exit tube;
said exit tube is adapted to extend said process material flow to a user destination.
Preferably said pickoffs are electro-magnetic devices each having a magnet and a coil.
Preferably said pickoffs each comprises a light source and an optical detector with the magnitude of the Coriolis deflection of said single flow tube defining the magnitude of the output current of said optical detector.
Preferably said massive base has a pair of upwardly extending parallel side walls having coaxial openings through which said single flow tube and said return tube extend.
Preferably said massive base is substantially u-shaped.
Preferably said massive base is a solid rectangular element defining a parallelepiped;
said single flow tube is mounted to upwardly extending posts affixed to a surface of said massive base.
Preferably ends of said single flow tube and said return tube extend beyond the outer surface of each leg.
Preferably said single flow tube is substantially straight.
Preferably Coriolis flowmeter comprises a second flow tube coupled to said massive base to define a dynamically balanced structure when vibrated by said driver while containing said process material.
Preferably said driver is positioned when in use on a top surface of said single flow tube.
Preferably a dynamic balancer means coupled to said massive base proximate said nodes to reduce the vibration of said nodes.
Preferably said dynamic balancer means is an active dynamic balancer controlled by the exchange of signals with said meter electronics.
Preferably said first and second flow tubes have an irregular shape.
Preferably the mass of said massive base is at least 100 times the mass of said flow tube with material flow.
Preferably the mass of said massive base is at least 1000 times the mass of said single flow tube with material flow.
Preferably said driver vibrates said flow tube at a resonant frequency of said material filled flow tube.
Preferably said driver vibrates said flow tube at a non resonant frequency of said material filled flow tube.
Preferably said Coriolis flowmeter is adapted to extend a flow of corrosive material including nitric acid.